Origin of the 6.85 $\mathsf{\mu}$m band near young stellar objects: The ammonium ion (NH4^+) revisited

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We have investigated whether the v 4 feature of NH + 4 is a viable candidate for the 6.85 μm absorption band seen towards embedded young stellar objects. To produce NH + 4 astrophysical ice analogs consisting of H 2 O, CO 2 , NH 3 and O 2 were UV photolysed. The IR spectra reveal peaks that are identified with the NH + 4 , NO - 2 , NO - 3 and HCO - 3 ions. It is shown that the NH + 4 matches two absorption features that are observed towards embedded young stellar objects, i.e., the strong 6.85 μm feature and the 3.26 μm feature. The characteristic redshift with temperature of the interstellar 6.85 μm feature is well reproduced. The abundance of NH + 4 in interstellar ices would be typically 10% relative to H 2 O. The experiments show that the counterions produce little distinct spectral signature but rather a pseudo-continuum if a variety of them is present in a H 2 O dominated environment. The anions could therefore go undetected in IR spectra of interstellar ice. In the ISM, where additional mechanisms such as surface chemistry and additional elements such as sulfur are available many acids and an even wider variety of anions could be produced. These components may be detectable once the ices sublime, e.g., in hot cores.